Axis 670 Manual de Instruções descarregar pdf (Página 15)

The summing amplifier algebraically sums all four of these signals to obtain a composite signal

that is sent to the output stage. During startup, a FET, Ql is tumed on slowly so that the servo

has time to stabilize in a contiolled manner. Also, when an ertor condition is sensed, an analog

switch, U15C, is shorted across this amplifier, thereby opening the loop and shorting the signal

being sent to the output stage to ground. During mirror alignment the gain of the summing

amplifier is reduced by two orders of magnitude which drastically lowers the loop gain. This

allows the customer to align the mirror manually without the servo going unstable. This is

explained

fiirther

in the mirtor alignment section in appendix 6.0.

3.7.2. Class 0

Our class 0 servo consists of

the

following circuits: a position differentiator, an error amplifier,

a cmrent integrator, and a summing amplifier. The tiansfer fimction can be characterized by the

following differential equation:

V(t) = Al • d (Vpos(t)) + A2 • Error(t) + A3 •

I(t)dt

where:

V(t) = output of summing amplifier

Al -

= coefficients that are adjusted with the tuning pots

R25,

R28,

and

R59.

Error(t) = the error signal generated as the difference between the position signal, Vp, and the

output ofthe slew rate limiter (command signal).

I(t) = the current

flowing

through the rotor coil.

The position differentiator takes the first derivative of the position signal to yield angular

velocity. This velocity signal is one of two sources of damping for the

servo.

Its -3db bandwidth

is set relatively low. This is to prevent high frequency noise, present on any differentiated

signal, from entering the summing amplifier. Its contribution to the servo is to provide damping

at low fi^uency and is contiolled

R25.

The error amplifier compares the actual position to the commanded position and generates a

signal proportional to this error. Since this is not an integrated signal, the bandwidth of this

stage is much higher. Thus, the closed-loop bandwidth of the servo is also higher. The sacrifice

is that if

there

is any fiiction or spring present, there will be some DC error. However, since

Cambridge Technology's scanners have very low friction and no torsion bar, this error is quite

small. The contribution of this amplifier to the servo response is contiolled

R28.

The current integrator produces a signal proportional to the integral of the current flowing

through the rotor. Since the current flowing through the rotor is proportional to the torque

produced, it is also proportional to the angular acceleration. Thus, the integral of current can be

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